I've been using a 21" Halliburton poly zero roller as my carryon for the last 4 years. It's the two-wheeled version as I was afraid that the 4-roller would be to big for some overheads (Euro flights in particular).
This is the bag I have (in silver):
FWIW, I'm C-level in a global company and travel a "fair" amount, and this thing has been awesome. Lightweight and flexible enough...